The present invention relates generally to apparatus for use in digital electronic circuits, and, more particularly, to a transmission line network for driving circuit components. Still more particularly, the present invention is directed to an improved transmission line network for delivering low distortion digital signals with relatively uniform rise times to multiple, distributed capacitive loads.
According to present practices, integrated circuit ("IC") packages are mass-produced and installed in electronic circuits within highly sophisticated, complex, and costly equipment. As with many mass-produced products, IC packages occasionally are prone to failure, in many cases at the beginning of operation. The complexity of the equipment within which such packages are installed makes post-installation failures highly undesirable. For example, when equipment reaches the final inspection stage of production, before failures are detected, the high level skills required for testing and repair add a significant cost to production expenses. Even more significantly, when the product has been installed in the field and a service technician must make repairs, the costs thereby incurred can have a significant effect on profitability. As a result, manufacturers of electronic equipment are demanding ever greater quality and dependability in commercial grade IC packages.
Quality and dependability are enhanced substantially by early detection of those IC packages likely to fail in the first few hours of operation. Virtually all IC packages manufactured today, before they are sold for use in electronic equipment, are "burned-in" for a predetermined period of time and then tested to detect IC packages that are defective or otherwise liable to fail prematurely.
The burn-in process includes (i) placing the IC packages in sockets arranged in arrays on printed circuit boards ("burn-in boards"); (ii) placing the burn-in boards with the packages, or devices under test ("DUTs"), mounted thereon in a chamber whose environment, particularly temperature, is controllable; (iii) applying direct current ("dc") biases to each package on each board in such a manner as to forward and reverse bias as many of the package's junctions as possible, and/or actively clocking each package up to its maximum rated conditions, such application of dc biases and clock signals being accomplished substantially simultaneously to each DUT on a burn-in board; (iv) removing the burn-in boards from the chamber after the IC packages have been subjected to the environmental condition of the chamber and the biases and clock signals for a designated period of time; and (v) removing the IC packages from the burn-in boards. The present invention focuses primarily on step (iii), which may also include the simultaneous application of test signals to the DUTs, with monitoring of the results of such testing.
A conventional burn-in board includes a plurality of sockets arranged in rows and columns. Signal lines, or traces etched onto the printed circuit card comprising the burn-in board, connect each pin of each socket to a tab on the connector edge of the board. Typically, one tab on an edge connector connects to one particular pin on each socket. The signal line between the tab and a socket pin may be modeled as a portion of a transmission line.
Each such transmission line typically "fans out" in several branches to a plurality of DUTs. One transmission line, for example, may be subdivided into a major bus line with a plurality of parallel subsidiary transmission lines branching off of the major bus line. The major bus line may extend along one end of the burn-in board, and the subsidiary transmission lines, in such case, may correspond to each row of sockets on the burn-in board, with the sockets of each row distributed serially along a subsidiary transmission line. In order to inimize the amount of power required to be delivered by the driver, the transmission lines connecting the driver to each socket typically are not terminated with the characteristic impedance of the transmission line.
A conventional burn-in system or burn-in and test system includes an environmental chamber having means for receiving a plurality of burn-in boards. The connector edge on each burn-in board is received within an edge connector, preferably outside the burn-in chamber. The opposing side of the edge connector may be attached to a second printed circuit card, or driver board, or it may be attached to a printed circuit backplane assembly, which also has edge connectors for receiving driver boards, as shown and described in U.S. Pat. No. 4,374,317.
The driver boards generate digital signals for exercising and testing the DUTs on the burn-in boards within the burn-in chamber. The digital signals are propagated through the edge connector and along parallel transmission lines on the burn-in board to the target pin on each DUT. Because the transmission lines typically are not terminated with the characteristic impedance of the transmission lines, signal reflection from the far end of the signal line, as well as from distributed loads, tends to cause distortion of signals propagated along the transmission lines. This distortion includes overshoot near the far end of the lines and a pause or. "step" in the rising or falling edge of the signal near the starting end of the lines, more precisely a period in the rising or falling edge during which the slope of the signal approaches zero or actually reverses before resuming its rise or fall to the intended level.
Signal distortion may also be caused by cross-talk between the plurality of transmission lines. Each DUT on a burn-in board typically includes a plurality of pins, each of which may connect to a different transmission line. Because space is limited on a burn-in board, the plurality of parallel transmission lines are positioned in close proximity to one another. As a consequence, capacitive and/or inductive coupling between adjacent transmission lines may cause waveshape distortion of signals propagating along the transmission lines.
Similar problems exist in transmission line networks other than the network connecting drivers to sockets on a burn-in board. Outside the burn-in chamber, for example, a network of parallel transmission lines may connect a pattern generator to a plurality of driver boards. The transmission line network may comprise a ribbon cable or a printed circuit board, and the driver boards may be distributed serially along the transmission lines. Because of the close proximity of the parallel transmission lines and the absence of a circuit terminating the transmission lines with their characteristic impedance, signals propagated along the network experience waveshape distortion similar to that discussed in connection with the burn-in board transmission lines.
Waveshape distortion such as that discussed herein can cause a number of problems that tend sometimes to invalidate the results of the testing and exercising to which the DUTS are exposed in the burn-in chamber. These problems include, for the step distortion described above, double triggering of the DUT by the nonuniform slope of the rising or falling edge of the signal and, for overshoot, latch-up of the DUT during which the DUT might be destroyed.
Thus, it would be desirable to provide a transmission line network for delivering a plurality of signals to a plurality of capacitive loads in which the transmission lines are not terminated with their characteristic impedance and the signals propagated along the transmission lines have relatively uniform rise or fall times and waveshapes not distorted by excessive overshoot or cross-talk.